1. Field of the Invention
This invention relates to an ultrasonic apparatus provided with an autofocusing function.
2. Description of the Prior Art
Heretofore, in defect inspection of electronic components and the like by ultrasonic waves, one practical system uses a so-called pulse-echo method in which, for the purpose of increasing the resolution, ultrasonic pulses are emitted toward an object to be inspected in water using a focusing-type probe which focuses an ultrasonic beam into a narrow beam, and nondestructive inspection of the object is performed by analyzing signals reflected from the object. This method is described in detail in, for example, Japanese Patent Public Disclosure (Kokai) No. 63-47658 (1988).
In this method, the focus of the ultrasonic beam must exactly coincide with a desired depth to be inspected. The focal length of the probe is usually fixed, and focusing is performed by changing the distance between the probe and the object to be inspected. If the depth to be inspected within the object from the surface of the object is represented by d, the velocity of sound in water by V.sub.w, the velocity of sound within the object to be inspected by V.sub.s, and the focal length of the ultrasonic beam in water by F, then the target distance W.sub.p in water from the probe to the object to be inspected is expressed by EQU W.sub.p =F-d.multidot.V.sub.s /V.sub.w.
Hence, it can be understood that the probe should be disposed at a position which is at a distance equal to the target distance W.sub.p in water from the surface of the object to be inspected.
In general, however, the velocity of sound V.sub.s within the object to be inspected is unknown, and the velocity of sound V.sub.w in water and the focal length F in water change mainly due to changes in water temperature. Moreover, there exists an inherent deviation from a nominal value in the focal length F in water for every probe.
Accordingly, in the conventional method, it is necessary to separately measure or estimate the above-described values before starting inspection. Furthermore, since there exists no means for automatically adjusting the position of the probe to the target length W.sub.p in water, the operator himself must perform any adjusting operation while measuring the distance in water. Consequently, this inspection method is troublesome, causes many misoperations, and is not an efficient method.